1. Field of the Invention
The present invention relates, generally, to a method for preparing core-shell nanoparticles, comprising dispersing core nanoparticle powder supported on carbon in ethanol, adding a metal precursor which forms a shell and hydroquinone thereto, and mixing and reducing the same. In particular, the core-shell nanoparticles prepared in accordance with the present invention may be usefully used as catalysts and electrode materials of fuel cells.
2. Description of Related Art
A fuel cell, which transforms chemical energy resulting from the oxidation of fuel directly into the electrical energy, is considered to be the next-generation energy source. Particularly, in automobile-related fields, research is actively carried out because fuel cells have advantages in improved fuel efficiency, reduced emission, environment friendliness, or the like.
Proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), which are representative fuel cells, use catalyst materials prepared by dispersing nanoparticles of platinum or transition metal alloy including platinum on a carbon support as electrode materials. However, since the typical electrode material platinum is expensive and its reserves are limited, research has been actively carried out to considerably reduce the use of platinum or to replace it with other material. As a way of reducing the use of platinum, a method of forming a core-shell structure by coating platinum only on the surface of core metal has been proposed. The core-shell structure is reported to have enhanced activity of oxygen reduction since the surface alloy of the core metal with platinum weakens the strong binding between pure platinum and anions and reaction intermediates (OH) formed during the oxygen reduction (Stamenkovic, V. R. etc., Science, vol. 315, p. 493).
Coating of the dissimilar metal on the metal nanoparticle surface may be accomplished by chemical reduction or electrochemical coating.
Preferably, the chemical reduction may be performed by preparing and drying core nanoparticles and then coating surface metal thereon in another solution, or by adding a surface metal reagent in the same synthesis solution without drying to selectively reduce the surface. Among the surface alloys, coating of transition metals including platinum on gold nanoparticles has been studied the most. Gold is basically hydrophobic and forms very weak or no bonding with organic substances including oxygen when compared with other transition metals. Thus, gold particles have high surface energy in water because water molecules are hardly adsorbed thereon. For this reason, other transition metals may be coated well on the gold particles in water. Japanese Patent Laid-Open No. 2005-248298, incorporated by reference in its entirety herein, discloses a method of coating copper on silver core particles by reducing a silver precursor and a copper precursor sequentially by heating. However, research has not focused on methods of coating a less reactive metal on particles of highly reactive precious metal, e.g. coating of platinum on nickel or palladium, as yet.
In certain examples, the electrochemical coating may be suitably performed by reducing copper ion dissolved in an electrolyte solution by controlling the voltage of an electrode and then allowing platinum to be coated due to the reduction potential difference of platinum ion and copper ion. For instance, Korean Patent Publication No. 2008-045155, incorporated by reference in its entirety herein, discloses a method of coating copper on platinum nanoparticle core and then replacing the copper with gold. However, this method is inapplicable to metals having a reduction potential lower than that of copper. In addition, copper may remain without being completely replaced by platinum and thus act as an impurity. There is a method of reducing cobalt nanoparticles and then coating allowing platinum to be coated due to reduction potential difference, which is based on the same principle. However, this method is associated with a problem of core metal loss since platinum ion is reduced and coated on the surface in exchange for the dissolution of surface atoms.
The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.